Method for manufacturing a bipolar transistor having a polysilicon emitter

ABSTRACT

In the inventive method for manufacturing a bipolar transistor having a polysilicon emitter, a collector region of a first conductivity type and, adjoining thereto, a basis region of a second conductivity type will be generated at first. At least one layer of an insulating material will now be applied, wherein the at least one layer is patterned such that at least one section of the basis region is exposed. Next, a layer of a polycrystalline semiconductor material of the first conductivity type, which is heavily doped with doping atoms, will be generated such that the exposed section is essentially covered. Now, a second layer of a highly conductive material on the layer of the polycrystalline semiconductor material will be generated in order to form an emitter double layer with the same. Thereupon, at least part of the doping atoms of the first conductivity type of the heavily doped polycrystalline semiconductor layer is caused to get into the basis region to generate an emitter region of the first conductivity type.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of copending InternationalApplication No. PCT/EP02/08234, filed Jul. 10, 2002, which designatedthe United States and was not published in English.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to the manufacture of semiconductordevices and, in particular, to the manufacture of bipolar transistorshaving a polysilicon emitter, which comprise reduced emitter resistance.

[0004] 2. Description of Prior Art

[0005] In bipolar transistors, which are designed to take high powersand speeds, use is already made of polysilicon emitters. In thisconnection, with respect to the theoretical and experimental aspects ofthe use of bipolar transistors having a polysilicon emitter, referenceis made to the article by C. R. Selvakumar, “Theoretical andExperimental Aspects of Polysilicon Emitter Bipolar Transistors”,published in IEEE Press, 1988, pages 3 to 16.

[0006] Therefore, one embodiment of a bipolar transistor having apolysilicon emitter provides a heavily doped polysilicon layer locatedabove the basis, which serves both as a diffusion source for thegeneration of a flat (emitter/basis) semiconductor transition and as ameans for contacting the flat emitter region. After performing theconventional processing steps for manufacturing the basis region and theemitter window openings, either non-doped or doped polysilicon will beapplied, into which, if the polysilicon is non-doped, an exact quantityof arsenic atoms will be implanted. Thereupon, by way of heat treatment(tempering), damages will be annealed, and the emitter/basissemiconductor transition is formed.

[0007] As can be seen from the above-cited article on page 4, one of thecritical processing steps in the manufacture of bipolar transistorshaving polysilicon emitters consists of the treatment of the waverexactly before applying the polysilicon. Therefore, the many differenttreatment methods, as are known in the state of the art, may be roughlysub-divided into two categories. The first treatment refers tointentional or unintentional growth of a thin oxide layer (0.2 to 2 nm).The second treatment refers to the epitaxial growth of a thin thermalnitride layer (approximately 1.0 to 1.5 nm). The “interface” treatmentis important, since the same has strong effects on the electricalcharacteristics of bipolar transistors having a polysilicon emitter.

[0008] As has been mentioned above in brief, it is tried to achievebipolar transistors having high cut-off frequencies and high currentgains by forming the emitter(s) of a bipolar transistor by depositing aheavily doped polysilicon layer. The doping agent in the polysiliconlayer will then diffuse, by way of tempering, from the polysilicon layerinto the single crystal silicon substrate below, where it forms theelectrically-active emitter area of the bipolar transistor. Here, thepolysilicon used serves as a doping agent source, as a feed and also asa landing surface for the contact terminal holes yet to be formed. Asfor the operational properties of the transistor, the use of polysiliconhas the following decisive advantage that the interface between thepolysilicon layer and the single crystal silicon substrate serves as adiffusion barrier for minority carriers which are injected from thebasis, thus clearly increasing current gain and cut-off frequency of thetransistor.

[0009] However, one disadvantage of polysilicon is the specificresistance which is by orders of magnitude higher as compared to metals.The relatively high emitter resistance resulting therefrom especiallyaffects the high-frequency properties of the bipolar transistors. Owingto these problems, it has been tried to use as thin a polysilicon layeras possible. On the other hand, a certain minimum thickness of mostlyfar above 100 nm is required, since an etching of contacting holes forthe contact pads has to stop on this polysilicon layer in order toensure the process safety during the manufacture of bipolar transistors.The problem concerning the emitter resistance still increases withmodern bipolar transistors having very narrow emitter windows, since thepolysilicon used may completely fill the emitter window in this case,and, thus, the height of the polysilicon layer over the active emitterfurther increase

[0010] It should be appreciated that, instead of polysilicon, anamorphous silicon may also be used which, in turn, may rest tocrystallize in subsequent tempering processes.

[0011] In order to solve the problems shown above concerning themanufacture of bipolar transistors having a polysilicon emitter,concepts haven been taken up, which provide a thermal silicidation ofthe emitter after depositing a metal layer. Silicides are metal/siliconcompounds, which are used in silicon technology as temperature-stable,low-resistance traces and contacts. The silicide layers typicallyprovide a thickness of 0.1 to 0.2 μm. However, the silicide layer formedin this manner is, as a rule, relatively irregular, thus, in practice,making it impossible to fill the emitter window with this layer.

[0012] As a further measure, the layer thickness of the polysilicon waskept as low as possible and the doping of same was kept as high aspossible. If possible, filling the emitter window with polysilicon wasavoided, which, however, in earlier technologies, was much easier owingto larger emitter dimensions. If, after depositing the polysilicon onthe emitter, a very narrow gap is left, increased efforts werenecessary, depending on the technology chosen, when etching thecontacting holes, since this gap may be filled with an undesiredmaterial, e.g. with a nitride barrier, when depositing further layers.

[0013] In many cases, the negative influence of the emitter resistor onthe transistor properties was simply accepted and/or it was tried tocompensate for this negative influence in terms of circuit technology.

SUMMARY OF THE INVENTION

[0014] It is the object of the present invention to provide an improvedmethod for manufacturing a bipolar transistor having a polysiliconemitter whose emitter resistance is clearly reduced in order to improvethe electrical properties of the bipolar transistor.

[0015] In accordance with a first aspect, the invention provides methodfor manufacturing a bipolar transistor having a polysilicon emitter,having the following steps: generating a collector region of a firstconductivity type and a basis region of a second conductivity typeadjoining thereto in a semiconductor substrate; applying apolycrystalline layer of a polycrystalline semiconductor material of thesecond conductivity type heavily doped with doping atoms on thesubstrate, so that a portion of the basis region is exposed; applying atleast one insulating layer of an insulating material on thepolycrystalline layer; patterning the at least one insulating layer suchthat at least one section of the basis region is exposed; generating afurther polycrystalline layer of a polycrystalline semiconductormaterial of the first conductivity type heavily doped with a doping atomsuch that the exposed section is essentially covered, wherein thepolycrystalline layer and the further polycrystalline layer are isolatedby the insulating layer; generating a highly conductive layer of ahighly conductive material on the further polycrystalline layer togenerate an emitter double layer with the same; effecting that at leastpart of the doping atoms of the second conductivity type of thepolycrystalline layer get into the semiconductor substrate toelectrically connect the base region to the polycrystalline layer;effecting that at least part of the doping atoms of the firstconductivity type of the heavily doped further polycrystalline layer getinto the basis region to generate an emitter region of the firstconductivity type; structuring the emitter double layer for generatingan emitter terminal area; contacting the emitter terminal area with ancontact terminal.

[0016] The present invention is based on the recognition that, byforming an emitter double layer during the manufacture of a bipolartransistor having a polysilicon emitter, the specific resistance of theemitter terminal will be reduced and, thus, the electricalcharacteristics of the device will significantly improve with thepresent invention, the emitter of the bipolar transistor will bedeposited in two stages. Here, the first layer consists of a common,heavily doped polysilicon material. This polysilicon layer now stillonly serves as source for the doping material and for generating apolysilicon single crystalline interface between a polysilicon layer andthe single crystal semiconductor material of the substrate. As a result,the polysilicon layer used may be selected significantly thinner as hasbeen the case so far. The second layer applied is a layer of a highlyconductive material, by way of which the lead resistance to the emitterof the bipolar transistor is kept at a low level. This highly conductivelayer further serves as a stop layer for the etching of the contactingholes to be performed for the various contact pads. This layer maycompletely fill the emitter window without having any considerablynegative effect on the emitter resistance, i.e. without the emitterresistance being increased.

[0017] This second highly conductive layer has to sustain the hightemperatures of emitter tempering (of the temperature treatment) oftypically about 1000° C. or higher and, for reasons based on themanufacturing technology, should further provide properties similar tothe silicon material used in the various manufacturing processes, suchas e.g. in dry etching processes.

[0018] By way of the inventive method for manufacturing a bipolartransistor having a polysilicon emitter, in which a two-layer emitterdeposition is provided in order to form an emitter double layer, whichensures an extremely low emitter resistance, extremely favourableelectrically characteristics of the transistor may thus be achieved. Asa result, the reduction in the emitter resistance achieved by thedeposited emitter double layer has a positive impact on the cut-offfrequency and, in general, also on the voltage and power gain in acircuit.

[0019] As already noted, the first, lower layer consists of apolysilicon material, which is effective as a doping agent source forthe active transistor region, wherein the second, upper layer consistsof a highly conductive material, which serves as an etch stopping meansfor etching the contacting holes of the contact pads as well as for thevertical current transport between the contact pads and the siliconemitter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] These and other object and features of the present invention willbecome clear from the following description taken in conjunction withthe accompanying drawing, in which:

[0021]FIG. 1 shows an interim state of the manufacturing process of abipolar transistor having a polysilicon emitter having a narrow emitterwindow after depositing the emitter polysilicon material and thesilicide layer; and

[0022]FIG. 2 shows the state of the manufacturing process of a bipolartransistor having a polysilicon emitter having a narrow emitter windowafter patterning the emitter double layer, after tempering andcontacting.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] With reference to FIGS. 1 and 2, a preferred embodiment of thepresent invention for manufacturing a bipolar transistor having apolysilicon emitter will now be explained in detail.

[0024] As shown in FIG. 1, preferably, a single crystal silicon body isused, which serves as a substrate 10 for the bipolar transistor. In thesubstrate 10, a first region 12 of a first conductivity type is formed,with this region 12 being referred to as a collector region below. Inthe substrate 10, a further region 14 of a second conductivity type isfurther formed, which is herein referred to as a basis region 14 below.

[0025] In connection with the present invention, the first conductivitytype designates a so-called n-type doping, while the second conductivitytype designates a so-called p-type doping. A doping in a semiconductormaterial is referred to as a n-type, if the majority charge carrierstherein are electrons, wherein a doping in a semiconductor material isreferred to as a p-type, if the majority charge carriers therein areholes. In the present invention the conductivity types of the dopingsmay each be selected vice versa.

[0026] The basis region 14 adjoins the collector region 12, wherein atleast one section of the basis region 14 is formed between the surface15 of the substrate 10 and the collector region 12. On the surface ofthe substrate 10, a polycrystalline layer 17, e.g. of polysilicon, willbe applied in an appropriate manner, which provides the secondconductivity type (p-type), wherein the basis region 14 in the substrate10 remains essentially exposed. In the following, this layer 17 servesas a p-doped basis terminal area for the basis region 14.

[0027] On the surface of the substrate 10 or therein, one or more layers16 of a material, e.g. a dielectric (insulating) material, are formed,wherein the dielectric layers are patterned such that at least onesection of the basis region 14 is exposed.

[0028] Next, a layer 18 of a polycrystalline semi-material, preferablysilicone, will be applied such that this polysilicon layer 1Bessentially covers the exposed section of the basis region 14.

[0029] Since non-doped polysilicon layers provide a very high resistance(approximately 10⁴ Ωcm), in the present case, the polysilicon layer 18,since the same has an electrically conductive function in thetransistor, will be provided with doping agents, e.g. boron, phosphor orarsenic, in order to achieve the respective doping type, the desireddoping strength and, thus, the desired electrically conductivity of thepolysilicon layer. In order to save an additional doping step, thedoping of the polysilicon layer 18 is generally achieved during thepolysilicon deposition by adding suitable materials. In the presentcase, the polysilicon layer 18 comprises the first conductivity type(n-type).

[0030] With the present invention, an already heavily doped polysiliconmaterial is preferably applied, since a further, second layer 20 of ahighly conductive material will be applied directly onto the existingpolysilicon layer 18, in order to form, together with the polysiliconlayer 18, a so-called emitter double layer. The second layer 20consisting of a highly conductive material is normally a silicide layer.Silicides are metal/silicone compounds, which are used astemperature-stable low-resistance materials in silicone technology.These suicide layers typically provide a thickness of 0.1 to 0.2 μm,wherein a thickness of 0.1 to 0.2 μm refers to the thickness depositedon regular faces. Therefore, in the emitter window, the thickness orheight of the silicide layer lies clearly above 0.2 μm, e.g. at 0.5 μm.Most frequently, silicides such as MoSi₂ or WSi₂ are used.

[0031] For the explanation of the further steps of the inventive methodfor manufacturing a bipolar transistor having a polysilicon emitter,reference is now made to FIG. 2. The present semiconductor structurewill now be subjected to a temperature treatment (tempering), such thatat least some of the doping agents from the heavily doped polysiliconlayer 18 diffuse into the single crystal body, i.e. into the substrate10. As a result, the active emitter area 22 forms in the substrate, i.e.especially adjacent to the basis area 14. Thus, at least some part ofthe doping atoms of the first conductivity type of the heavily dopedpolysilicon layer 18 gets into the substrate, to generate, adjacent tothe basis region 14 in the substrate 10, an active emitter region 22 ofthe first conductivity type. The active emitter region 22 extends fromthe interface 15, between the polysilicon layer 18 and the substrate 10,into the semiconductor material of substrate 10.

[0032] Furthermore, during temperature treatment, some part of thedoping atoms of the second conductivity type of the polysilicon layer17, which is doped with this second conductivity type and which has beenprovided for the basis terminals 15, gets into the substrate 10, with alarge-surface connection to the basis region 14 in the substrate 10resulting therefrom.

[0033] The term temperature treatment, or tempering, in silicontechnology refers to the treatment of silicon at increased temperaturesin an inert atmosphere, e.g. nitrogen, argon, hydrogen, and forming gas.As a result, no new layers are grown and no material will be removed,but the layers already existing and the silicon substrate itself will besubjected to decisive changes. In the present case, the doping agents ofthe first or second conductivity type of the various, differently dopedpolysilicon layers 17, 18 get into the adjoining semiconductor materialof the semiconductor substrate 10.

[0034] Next, the emitter double layer consisting of the polysiliconlayer 18 and the highly conductive second layer 20 will be patterned togenerate an emitter terminal area of the bipolar transistor. Patterningis usually effected by dry etching of the respective layers. Processingis facilitated, if the upper, highly conductive silicide layer 20essentially comprises the same or comparative processing properties,e.g. etching properties, such as the polysilicon layer 18.

[0035] The exposed sections on the remaining present semiconductorstructure are now usually filled with an encapsulating insulatingmaterial 28. Further, the so-called contacting holes will be etched inorder to provide the contact terminal 24 for the emitter terminal areaand the contact terminals 26 for the basis terminal area 17. In theemitter terminal area, the silicide layer 20 serves as an etch stoppingmeans for etching the contacting holes.

[0036] By way of the above-described inventive manufacture of a bipolartransistor having a polysilicon emitter, it is possible to clearlyreduce the specific resistance of the emitter terminal area, whereby theelectrical characteristics of a bipolar transistor may be considerablyimproved. As a result, the reduction in the emitter resistance achievedby way of the emitter double layer deposited has a positive effect onthe cut-off frequency and, in general, also on the voltage and powergain in a circuit.

[0037] The inventive, advantageous concept for manufacturing a bipolartransistor having a polysilicon emitter essentially comprises the stepof performing the deposition of the emitter terminal area in two stages.As a result, the first layer 18 consists of the usual heavily dopedpolysilicon material. In the present invention, it merely serves as asource for the doping agent and for generating the polysilicon singlecrystal interface and may, therefore, be selected thinner than before.The second layer 20 is a layer of a highly conductive material whichkeeps the lead resistance at a low level and which serves as a stoppinglayer for etching the contacting holes. It can fill the emitter windowwithout significantly increasing the emitter resistance. In thepreferred processing, this second layer 20 has to sustain the hightemperatures of emitter tempering, which are typically about 1000° C. ormore, and, for simplifying the processing in dry etching processes, mayprovide comparative properties such as the silicone material.

[0038] In principal, a type of processing is also conceivable in whichemitter tempering occurs before depositing the second layer. Whendepositing the second layer, both pure metals and metal siliconcompounds (silicides) are ideal, wherein, in particular, all silicidesof high-melting-point metals, such as e.g. tungsten-disilicide andmolybdenum-disilicide, are used. For other materials, such as e.g.tungsten, an additional deposition of a diffusion barrier may benecessary.

[0039] In the inventive method for manufacturing a bipolar transistorhaving a polysilicon emitter, a two-stage emitter deposition will beperformed in accordance with the invention, with the lower layer ofpolysilicon being effective as a doping agent source for the activetransistor region and the upper highly conductive layer being effectiveas an etch stopping means for etching the contacting holes and also forthe vertical current transport between the contacting hole and thepolysilicon emitter.

[0040] Further, it should be appreciated that the present invention isalso applicable to deviating transistor architectures, in particular,those having an epitaxially grown basis area. Therefore, there aretransistor architectures, in which the basis area and sometimes alsopart of the collector area are epitatically grown onto the substrate. Inthese architectures, which, in the future, are very likely to be usedmore frequently, the inventive emitter double layer may also be used inan advantageous manner.

[0041] While this invention has been described in terms of severalpreferred embodiments, there are alterations, permutations, andequivalents which fall within the scope of this invention. It shouldalso be noted that there are many alternative ways of implementing themethods and compositions of the present invention. It is thereforeintended that the following appended claims be interpreted as includingall such alterations, permutations, and equivalents as fall within thetrue spirit and scope of the present invention.

What is claimed is:
 1. Method for manufacturing a bipolar transistorhaving a polysilicon emitter, comprising: generating a collector regionof a first conductivity type and a basis region of a second conductivitytype adjoining thereto in a semiconductor substrate; applying apolycrystalline layer of a polycrystalline semiconductor material of thesecond conductivity type doped with doping atoms on the substrate, sothat a portion of the basis region is exposed; applying at least oneinsulating layer of an insulating material on the polycrystalline layer;patterning the at least one insulating layer such that at least onesection of the basis region is exposed; generating a furtherpolycrystalline layer of a polycrystalline semiconductor material of thefirst conductivity type heavily doped with doping atoms such that theexposed section is essentially covered, wherein the polycrystallinelayer and the further polycrystalline layer are isolated by theinsulating layer; generating a highly conductive layer of a highlyconductive material on the further polycrystalline layer to form anemitter double layer with the same; effecting, via a temperaturetreatment, that at least part of the doping atoms of the secondconductivity type of the polycrystalline layer get into thesemiconductor substrate to electrically connect the base region to thepolycrystalline layer; effecting that at least part of the doping atomsof the first conductivity type of the heavily doped furtherpolycrystalline layer get into the basis region to generate an emitterregion of the first conductivity type; structuring the emitter doublelayer for generating an emitter terminal area; contacting the emitterterminal area with an contact terminal, wherein the layer and thecontact terminal vary, comprising the following substeps: applying aninsulating material on the emitter terminal area; and etching a contactvia into the insulating material, wherein the highly conductive layer inthe emitter terminal area is effective as stop layer for the viaetching.
 2. Method in accordance with claim 1, wherein the step ofeffecting that at least part of the doping atoms of the firstconductivity type of the heavily doped further polycristalline layergets into the basis region, will be performed by means of tempering. 3.Method in accordance with claim 1, wherein the highly conductive layerconsists of a material having comparative processing properties as thesemiconductor material.
 4. Method in accordance with claim 1, whereinthe step of effecting that at least part of the doping atoms of thefirst conductivity type of the heavily doped further polycrystallinelayer get into the basis region will be performed before or after thestep of generating the highly conductive layer.